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I just read http://www.bigclive.com/ignite.htm It shows igniting a 10ohm resistor with 12 volts. I was wondering, could a 10ohm resistor be ignited with less voltage(say, 9Volts), and if so, what is the minimum voltage?

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    \$\begingroup\$ depends on the wattage rating and the current applied. Watts = amps * volts \$\endgroup\$ – kenny Dec 13 '11 at 0:13
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    \$\begingroup\$ Also the resistor technology. Carbon ones will burn. Wire-wound ones will just fuse. \$\endgroup\$ – Majenko Dec 13 '11 at 0:15
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    \$\begingroup\$ Also the power source. a 12 volt car battery has an output impedance much lower than 10 ohms, so the resistor will have about 12 volts on it. A 9-volt "transistor" battery has an output impedance of tens of ohms, so the output voltage will drop a lot if you wire a 10 ohm load onto it. Also, the battery itself may overheat and burn. \$\endgroup\$ – Mike DeSimone Dec 14 '11 at 14:37
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DO NOT destructively test resistors or any other devices without the DIRECT SUPERVISION OF AN ADULT QUALIFIED TO INSURE SAFETY

It depends on the wattage rating of the resistor, airflow, atmospheric pressure and composition, what the leads are connected to internally and externally, the temperature coefficient of the resistor, and the degree to which its materials are flame retardant.

DO NOT destructively test resistors or any other devices without the DIRECT SUPERVISION OF AN ADULT QUALIFIED TO INSURE SAFETY

You can find out if you are exceeding the manufacturer's rating by comparing the square of voltage, divided by resistance, to the rated power limit in watts. For example, 9*9/10 or 8.1 watts is drastically more than the 1/4 watt rating of most small through hole resistors so it's clear that the component is being abused and there is a substantial risk that something bad and potentially dangerous will happen. (1/2 and 1/8 watt units are also common, though you can get higher rated ones).

DO NOT destructively test resistors or any other devices without the DIRECT SUPERVISION OF AN ADULT QUALIFIED TO INSURE SAFETY

And don't assume the resistor will just burn; it could also explode. Either one can be hazardous to your health and surrounding property. Also consider that it could be hot enough ignite other materials or produce nasty gases even if the resistor itself does not burst into flame. You could also overheat whatever is sourcing power to the resistor.

DO NOT destructively test resistors or any other devices without the DIRECT SUPERVISION OF AN ADULT QUALIFIED TO INSURE SAFETY

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  • \$\begingroup\$ Thanks! The resistor will be in a closed container, and it is designed to cause some sort of heat, even if it explodes. \$\endgroup\$ – DrAwesome Dec 13 '11 at 1:11
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    \$\begingroup\$ Exploding resistors may have other side effects, depending on composition. If there's a significant amount of carbon or conductive material, it can coat nearby surfaces with a thin layer of this conductive material (usually looks like a shiny, brown-tinted mostly-transparent coating). IIRC, this is known as "sputtering" in the chemistry community. \$\endgroup\$ – Mike DeSimone Dec 14 '11 at 14:24
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Well, if all you want to do is ignite fireworks (homemade or otherwise), then just use Estes Rocket igniters. Simple, reliable, relatively cheap, etc. Let someone else figure out all of the hard electro-chemical stuff.

Update:

Abstinence only education in the US has been a huge failure, and expecting abstinence here is not reasonable. I remember the things I did when I was 13, and the things my 12 year old son has done recently, and two things immediately come to mind: 1. I fear for Peapodamus' parents. 2. Peapodamus is going to do it, no matter what anybody says.

While my "inner lawyer" has to repeat what others have said, I must also give you practical advise for when you choose to ignore everybody else. Hopefully this advice will keep you, and the home you live in, safe. I am doing this fully realizing that others in this group will disagree with me for it, thinking that I'm somehow enabling kids to do dangerous things. But I'm not enabling-- since they are going to do it anyway-- just trying to keep people safe despite their "youthful passions".

So... Here's my advice. Don't do it. Failing that, start small. The smaller the better. You need to know how something will behave in the small scale before you go bigger. When you go bigger, do it gradually. Don't do something small and then go immediately to something big. Flammable stuff tends to behave differently at different scales.

Something combustible in a sealed container is called a bomb-- don't use sealed containers! Ironically, the harder/stiffer/stronger the container the more dangerous it can be. I cannot stress this enough. Aside from the obvious risks of explosions, a sealed container becomes very unpredictable, even hours or days after "the event". The container could also cause the whole thing to shoot across the yard, starting grass fires along the way. If that doesn't convince you, a sealed container could be the difference between a misdemeanor and a felony in some states. And you really don't want a felony! Well, you don't want a misdemeanor either, so just don't do it!

Don't light anything unless you are far away from anything flammable, like grass, cars, or your house. If you find yourself saying, "That should be far enough away", then you're not far enough away. Trust me on that one! I know from experience!

Never, ever, store "prepared compounds". If you mix up something, use it immediately. Don't mix up a big batch and save some for later. Also, be precise when mixing stuff up.

Have some sort of fire extinguisher ready, and know how to use it! There are different kinds of fire extinguishers and each has different nuances to how they are used. Also, have a backup to the fire extinguisher. A garden hose is a good backup, but make sure before hand that the hose will actually reach! Trust me on that one! I've learned that from experience too! :)

When doing stuff like this with electronics, always use battery power and never AC power. If, for some reason, you need to throw a bunch of water on something you do not want the added complication of getting electrocuted!

Finally, always have a friend recording the whole thing in video on their camera phone-- with an agreement that no matter what happens that video will be posted on YouTube. In that way, when you burn down half of your back yard the video can serve as a warning to others to not try it themselves. Also, before you do anything go watch the videos that others have made so you know what not to do.

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    \$\begingroup\$ Another super cheap alternative is to take a single steel wool thread and connect to wires. Those threads easily burn when connected to 9V. \$\endgroup\$ – hlovdal Dec 13 '11 at 7:51
  • \$\begingroup\$ Actually, some fine steel wool impregnated with slow burning gun powder should light just about anything, and is super easy to buy here in the US. \$\endgroup\$ – user3624 Dec 13 '11 at 14:03
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    \$\begingroup\$ There are reasons why pyrotechnic equipment is age restricted :-). Maturity is a significant issue in doing "funny" things with equipment that can possibly kill others or yourself or destroy property. Smoke generators and crackle bombs can kill or cause major damage if enclosed in sealed containers. It takes some skill to maim or kill yourself with these but people still manage to. \$\endgroup\$ – Russell McMahon Dec 13 '11 at 21:19
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    \$\begingroup\$ @peapodamous DO NOT destructively test resistors or any other devices without the DIRECT SUPERVISION OF AN ADULT QUALIFIED TO INSURE SAFETY \$\endgroup\$ – Chris Stratton Dec 14 '11 at 17:29
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    \$\begingroup\$ @peapodamus I edited my answer above to include lots of pragmatic safety stuff, just in case you can't find a qualified adult safety officer. :) \$\endgroup\$ – user3624 Dec 14 '11 at 18:15
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Added:

  • There are reasons why pyrotechnic equipment is age restricted :-) :-(.

    Note that smoke bombs and crackle bombs can kill or do major damage - especially when used in sealed containers or when remotely triggered in locations where one would be unlikely to light them by hand.

    Maturity is a significant issue in doing "funny" things with equipment that can possibly kill others or yourself or destroy property. Age is no guarantee of maturity but can be some guide. If nothing else, limiting access to dangerous material to older people ensures that people are old enough to be legally liable for the results of their actions.

    Smoke generators and crackle bombs can kill or cause major damage if enclosed in sealed containers. It takes some skill to maim or kill yourself with these but people still manage to do it.

If ignition is what you don't want, Murphy say it may happen any time you have Watts >> resistor rating, as here.

If ignition is what you WANT it is not so easy.
Many have tried to use resistors alone as igniters but reliability is usually not good. Some very careful designs may be acceptably good.

Within reason you can get a similar result by using the same dissipation at lower voltages.
So here, 12V + 10 R =~ 14 Watts.
So at 9V you want V^2/14 = 9^2/14 = 81/14 ~- 5.8 ohm
Use 5.6 ohm standard E12 value.

BUT this was probably a carbon resistor, the resistance value is achieved by cutting a spiral of variable pitch into a cylinder of carbon, so characteristics will vary - possibly importantly, possibly not.

If you want a more reliable igniter, find a resistor-style/voltage/resistance combination that ALWAYS [tm] gives an acceptable burst of high temperature, then surround it with powdered match head with an external wrapper of something suitable to keep them in close proximity, prevent loss of powdered material amd to keep out drafts.

Match heads contain Potassium Chlorate plus combustible material and are very easily ignited with a small burst of localised temperature (as most will have noticed). [When used on a standard matchbox the brown surface contains Red Phosphorus and a chemical reaction rather than elevated temperature of the match head alone is the ignition source].

Pyrotechnic igniters of this sort (contain chemicals) are becoming increasingly illegal.

Igniter operation is affected by ambient pressure. An igniter design that works reliably at ground level may frequently fail at high altitude where pressure is lower. Many amateur rocket ex-owners have discovered this to be fact while using igniters to attempt to deploy eg recovery parachutes. Lithobraking can happen.


Added after a discussion with Just Jeff:

A vast amount of effort has been expended by the Pyro community in making "bridgewire igniters".
The modern rules banning highly reactive chemical substances from such devices have made the task harder.
Reliability and repeatability don't come easily.

Jeff was suggesting that a wire as an ignter for a match may be as good as a resistor. I've personally found small wires rather hard to control BUT thereare almost certainly people able to use them well for this purpose. Often there is as much art as science in such things.

I have found that eg trying to make my own eg 250 mA fuses is exceedingly hard.
I think that for most people a resistor is liable to be a more repeatable and reliable match-igniter, than is copper wire. Nichrome or similar wire will work better (or, at least, more easily for mere mortals). I know wire can work - Somewhere over 4 decades ago I used to use multiple turns of cotton covered wire wound round match heads + a 12V car battery, as an igniter for things which I'm now surprised didn't kill or maim me. But I'd still recommend a resistor.


Resistance wire:

As well as Nichrome I also use "Constantan" wire for resistance wire purposes. Just under half the resistance of Nichrome for identical dimensions but about 50 times less variation in resistance with temperature change. About as available and a similar price.

Wikipedia - resistivity and conductivity. Includes tables

Nichrome. 80% Nickel. 20% Chrome. Wikipedia
Resistivity =~ 1.0 micro-ohm.meter

Constantan, 55% Copper. 45% Nickel. Wikipedia
Also called "Eureka"
Resistivity =~ 0.5 micro-ohm.meter

Interestingly - Iron wire has about 10% of the resistance of a dimensionally identical Nichrome wire at room temperature, but about 15 times as great a change in resistance with temperature.
Nichrome ~= 0.04%/K at 20 C Iron ~= 0.7% / K at 20 C Constantan 0.0008% / K at 20 C

[[Is use of degrees K and degrees C together "mixed units"? :-) ]]

http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity

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  • \$\begingroup\$ Thank you for the idea involving the match heads. If I had enough rep I would up-vote your answer :D You may as well know, it is all for a very funny prank, and it will be used to ignite smoke bombs and crackle bombs. \$\endgroup\$ – DrAwesome Dec 13 '11 at 1:21
  • \$\begingroup\$ @JustJeff - A vast amount of effort has been expended by the Pyro community in making "bridgewire igniters" The modern rules banning highly reactive chemical substances from such devices have made the task harder. Reliability and repeatability don't come easily. \$\endgroup\$ – Russell McMahon Dec 13 '11 at 1:51
  • \$\begingroup\$ I've found small wires rather hard to control Trying to make your own eg 250 mA fuse is exceedingly hard. A resistor is liable to provide a more repeatable and reliable solution as a match igniter than is copper wire. Nichrome or similar wire will work better.| I know wire can work - Somewhere over 4 decades ago I used to use multiple turns of cotton covered wire wound round match heads + a 12V car battery, as an igniter for things which I'm now surprised didn't kill or maim me. But I'd still recommend a resistor. \$\endgroup\$ – Russell McMahon Dec 13 '11 at 11:43
  • \$\begingroup\$ @JustJeff - OK - will do. I don't remmeber the fine detail but I don't recall your reply being "contentless". Many perpsectives go to make up reality and often enough (too often some times :-)) two apparently contradictory experiences occur and are worth noting. My heating fine wire experiences have not been stunningly successful - somebody else may have made an artform of using it. Such things happen. I find I can end up looking pugnacious when I'm really wanting to bat ideas around. New years resolution for me - be more careful when commenting on what people say :-). \$\endgroup\$ – Russell McMahon Dec 14 '11 at 11:36
  • \$\begingroup\$ hey, no, i didn't read you as pugnacious. i just asked myself 'do i want to make this into an answer', and since i told myself, 'nah, not really', thought it might be good to un-clutter your answer. \$\endgroup\$ – JustJeff Dec 14 '11 at 11:39

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